The invention relates generally to non-linear resistance elements and particularly to those used in devices for overvoltage protection in an electrical system.
Materials which have a distinctly non-linear current-voltage characteristic are commonly used in protective devices. One such protective device, for example, is an overvoltage, or surge arrester, used to protect electrical equipment against insulation damage resulting from voltage surges which exceed the normal operating voltage of the equipment. Surge arresters and their function are described to some extent in the following:
U.s. patent Nos.
3,671,800 ISSUED 20 Jun. 1972 to E. W. Stetson; 3,586,913 issued 22 Jun. 1971 to A. A. Olsen et al; 2,529,144 issued 7 Nov. 1950 to E. A. Evans et al.
Technical Publications
Electrical Transmission and Distribution Reference Book, Fourth Ed., Westinghouse Electric Corp, Pittsburgh, Pennsylvania, 1950 pp 621-627. Ichinose, Noboru "High-Performance Ceramic Varistor Element," in Japan Electronic Engineering, July 1972 pp 32-36.
Typically, an arrester includes a valve section and a gap section in series inside a cylindrical insulating housing. The valve section is made up of one or more disc-shaped non-linear resistance elements stacked face-to-face. The primary function of the valve section is to sufficiently reduce the magnitude of the follow-current after a discharge to permit interruption of the current by the gap section. The number and the dimensions of the resistance discs in a given arrester are determined by the protection requirements and manufacturing considerations.
Each of the resistance discs of an arrester is generally provided with an insulating collar about the periphery. The collar prevents flash-over between successive discs during a discharge of the arrester. In the manufacture of relatively porous fired silicon carbide discs which are widely used for arresters, a ceramic collar is formed by applying a water-based ceramic particle slurry to the peripheral surface of the silicon carbide material prior to firing. During the subsequent firing process, the glass particles in the slurry fuse together to form a tightly adherent insulating collar. For less porous discs a glass collar may be applied from a glass particle slurry in much the same way.
Certain zinc oxide compounds are superior to silicon carbide as the resistance disc material in that they have an even greater non-linear current-voltage characteristic. The zinc oxide compounds may be formed into resistance discs by pressing and sintering in much the same way as are silicon carbide discs. However, several problems are encountered in the application of a collar to zinc oxide discs. First, the nature of the pressed zinc oxide material is such that a water-based slurry does not sufficiently wet the surface prior to the sintering; therefore, the slurry must be applied after sintering and a separate heating cycle added to the manufacturing process to fuse the slurry particles together. Second, presently used slurries which fuse during the sintering process require a firing temperature of at least 800.degree.C (Celsius), so high that the characteristics of sintered zinc oxide discs are likely to be degraded by the addition of such a separate slurry firing step to the manufacturing. Thirdly, while certain common lead oxide glass slurries have a fusing temperature low enough to permit firing of the zinc oxide at that temperature to form a glass collar without significant degradation of the zinc oxide disc, the coefficient of thermal expansion of such glasses is so high relative to that of the zinc oxide compound that upon cooling of the disc the fused collar material fractures, rendering the collar defective.